In dentistry, addition silicones are the most widely used impression materials. Addition silicones cure with a hydrosilation mechanism and contain a platinum compound as a catalyst. Despite the addition of various surfactants, the hydrophilicity of the materials as measured by contact angle measurements, especially before set is completed, is very low. This reduces the ability of the impression material to displace oral fluids during curing and results in a compromised impression. Another class of impression material, the polyethers, as exemplified by IMPREGUM® (from 3M ESPE) are 2-part systems containing imine terminated polyether copolymers cured by reaction with a strong acid. However, these polyethers suffer from high rigidity, which is a property of crosslinked polyethers, and poor taste and smell due to the presence of imines and strong acids.
In related U.S. application Ser. Nos. 11/276,270 and 11/276,273 filed on Feb. 21, 2006 and referenced above, compositions containing a resin having a urethane polyester or polyether carboxylate backbone functionalized with at least two cycloolefin groups capable of undergoing a metathesis reaction are described. When used in a dental impression material, however, the composition may be too stiff and brittle, lacking the flexibility of pure addition silicone elastomers. To retain the desired hydrophilicity of a polyether while having the flexibility of an addition silicone, there is a need for a copolymer with both properties that can be used as a resin component together with the urethane polyester or polyether carboxylate.
One category of copolymers having good flexibility is the siloxane polyether (SPE) copolymers. There are many commercially available SPE copolymers. These block copolymers are of two types. One SPE copolymer type is prepared by the hydrosilation reaction of hydride functionalized polydimethylsiloxanes with alkenes. The hydrosilation prepared SPE copolymers contain Si—C—C linkages and can be named alkyl-SPE. There are several commercially available alkyl-SPE copolymers, such as Dow Corning 2-8692 fluid or the Silwet series from GE Silicones. There are many diverse uses for these copolymers, such as nonionic surfactants and defoamers. However the preparation method used for alkyl-SPE, namely hydrosilation, suffers from the disadvantage that it never goes to completion, always resulting in the starting materials being present in the final product, especially the alkene reactant in non-negligible amounts, e.g., at 10% or higher, and other unsaturated impurities. The presence of allyl ethers in the SPE does not generally interfere with many commercial uses of these products, such as for surfactants. But in the case of curing by metathesis reaction, such as ring opening metathesis polymerization (ROMP), the allyl ether impurities do interfere with the metathesis reaction by decreasing the activity of the metal carbene complex catalyst, possibly by irreversible binding. There is therefore a need for a preparation method for an SPE without unsaturated impurities being present in the final product.
Another SPE copolymer type is prepared by a condensation reaction, that is, by the coupling of a chlorine or acetoxy substituted polydimethylsiloxane (PDMS) with an alcohol to afford alkoxy substituted PDMS, but this method suffers from the difficulty of removing the hydrochloric acid or other acid waste and is relatively expensive to scale to large amounts. The condensation prepared SPE copolymers contain Si—O—C linkages and can be named alkoxy-SPE. The alkyl-SPE copolymers are more hydrolytically stable than the alkoxy-SPE in acid conditions. However in neutral or basic conditions their hydrolysis rates are comparable.
The dehydrogenative sylilation of hydride functionalized siloxanes and alcohols is a known synthetic route to highly pure alkoxy functionalized siloxanes (see, e.g., PMSE 2005, 92, 365; PMSE 2004, 91, 587). This dehydrogenation is carried out in the presence of a very strong Lewis acid catalyst, such as tris(pentafluoro-triphenyl)borane, B(C6F5)3. This preparatory route is convenient since the byproduct, hydrogen gas, is easy to remove as opposed to the chlorosiloxane route that gives difficult to dispose of hydrochloric acid.
The dehydrogenation of hydride-functionalized polyorganosiloxanes with alcohols has been described in U.S. Patent Application Publication No. US 2004/0186260 ('6260 publication) submitted by Goldschmidt A G. The process disclosed therein is for preparing alkoxy-substituted polyorganosiloxanes using the dehydrogenation reaction in the presence of a main group III and/or transition group III catalyst and optionally a solvent. Specifically, the '6260 publication contains examples relating to the reaction of hydride terminated (alpha, omega disubstituted and/or tethered (comb-like)) polyorganosiloxanes with simple alcohols and simple alcohol started polyethers. However, the '6260 publication does not disclose any specific copolymers or method of making same that provide the desired results in a dental impression material.
There is thus a need for an alkoxy-SPE copolymer with properties that can be used as a resin component together with the urethane polyester or polyether carboxylate for use in a dental impression material, and a process of making the same.